Various types of shock wave generators are known, operating especially by electrical discharges, piezoelectric effect, or detonations of chemical explosives. However, their prime cost is high and they are unsuitable for certain applications, especially for lithotrites used in medicine for the destruction of kidney stones.
The basic idea of U.S. Pat. No. 4,589,415 is to construct a lithotrite operating by shock waves which are transmitted strictly through the living tissues by a bundle of intermediate probes--without contact with the kidney stone to be destroyed--in such a manner that the energy density injected by each probe is low enough not to damage the tissues, while the sum of the energies concentrated on the stone must be high enough to destroy it. The possibility of using only one probe is therefore only theoretical, as the inevitable divergence of the waves, from the exit of the probe, implies a supplementary energy. The principal disadvantages of this technique arise from the experience acquired with the known extracorporal lithotrites. It is known, for example, that in order to be effective the concentration of the shock waves on the stone must take place within a volume of a few mm.sup.3, which can be contemplated here only on the basis of two very cumbersome visual display devices, forming a certain angle, in order to permit the precise orientation of the probes. It is likewise known that, in order to be effective, all the components of the shock wave must arrive "in phase" at the focus, which implies here, at the very least, a very lengthy and random trial and error experimentation. It is also known that, if the stone is not retained by the adjacent tissues, it will move under the effect of the shock waves, leading to the necessity to adapt permanently the orientation of the probes, with all the difficulties that this implies. Finally, the process experiences the same limits as the use of an extracorporal lithotrite, namely that the stone is no longer accessible when it is situated in the ureter, behind the pelvic ossature.
German Patent 278,700 (of 1913) describes a mechanism for placing and maintaining in vibration a membrane of an acoustic alarm, by means of a mechanism converting the rotary movement of a disk driven by a motor into an alternating translational movement of a component which is integral with the membrane, by virtue of a ball disposed between this component and this disk, in cells of these two parts. In this construction, there is no production of shock waves, but simply production of an acoustic signal.
French Patent 455,868 also describes a mechanical acoustic horn with a vibrating membrane, comprising a piston provided to execute a to and fro movement within a cylinder under the control of pneumatic means. This piston periodically pushes back the center of the membrane, which center is occupied by an appropriate anvil, in order to maintain its vibration. Not only would the production of shock waves be useless for a horn, but this possibility is excluded, the acoustic impedance of the membrane being matched to that of the air, which is 70,000 times smaller than that of the piston (let us recall that the acoustic impedance of a homogeneous medium is equal to the product of its specific gravity and the speed of sound in this medium).